1. Field of the Invention
The present invention relates to a machine tool with an automatic tool change function, which is capable of making engagement between a key of a tool spindle and a key-way of a tool or a tool holder without stopping rotation of the spindle.
2. Description of the Prior Art
Conventionally, in order to make engagement between a key of a tool spindle and a key-way of a tool, the spindle is stopped at a predetermined angular position where the key comes into the corresponding position of the key-way to thereby permit the tool to be inserted thereinto by a tool change arm.
In order to stop the spindle at such a predetermined angular position, two operational steps are required. That is, the spindle is first stopped within a predetermined angular range by controlling a spindle drive motor and then indexed to the predetermined angular position by a mechanical device including, for example, an index plate and an index pin. Accordingly, it takes a longer time for stopping the spindle at the predetermined angular position, resulting in a longer tool change time. Further, the device required for accomplishing the above operation is rather expensive.
In order to solve these disadvantages and perform a tool change operation without stopping rotation of a spindle, it has been considered to urge a flange portion of a tool toward the key of the rotating spindle to achieve the engagement between the key-way of the tool and the key of the spindle after relative frictional rotation therebetween. In this method, however, the contacting surfaces of the key and the flange portion of the tool are necessarily worn due to the sliding contact therebetween and the relatively high pressure applied thereto and there is a fear of decreasing the final accuracy after a machining operation due to the variation of the tool position inserted in the spindle as a result of uncertainty of which one of two key-ways is engaged with the key of the spindle.
Furthermore, even if a tool change operation is performed without stopping rotation of the spindle, there is a problem that a large diameter tool with a radially projected cutter has to be returned from the spindle to the tool magazine with its key-way being maintained at a predetermined angular position in order to prevent interference with an adjacent tool in the tool magazine.